RU2607410C2 - Dry sorbent introduction under conditions of non-stationary state into dry gas cleaning scrubber - Google Patents

Abstract

FIELD: fuel.

SUBSTANCE: invention relates to removal of solids and other contaminants from flue gases, produced during fuel combustion. Method of reducing emissions during fuel combustion, produced under operating conditions in non-stationary state in fuel combustion system, having gas passage path, which passes from fuel combustion chamber through spraying drier-absorber to bag filter, located downstream of spraying drier-absorber, wherein method includes: admixing of calcium hydroxide dry powder to carrier gas into gas passage path in input position, located downstream of fuel combustion chamber and upstream of bag filter, spraying of water into carrier gas in spray drier-absorber for moistening and carrier gas temperature reducing and calcium hydroxide powder deposition in bag filter for precipitate producing on filter, which reduces emissions during fuel combustion, wherein temperature in spray drier-absorber is less than 220 °F.

EFFECT: removal of solid particles and other contaminants from flue gases produced during fuel combustion.

24 cl, 7 dwg

Description

This application claims priority to the provisional US patent application Serial No. 61/540806, filed September 29, 2011. The description of this application in its entirety by reference is fully incorporated into the present description.

State of the art

The present invention generally relates to the removal of particulate matter and other contaminants from flue gases obtained during fuel combustion using a flue gas desulfurization system based on a dry gas scrubber during start-up, shutdown, malfunction and other abnormal periods. In particular, this invention relates to new and suitable methods and systems for collecting sulfur dioxide (SO ₂ ), sulfur trioxide (SO ₃ ), HCl and other acid gases by introducing a dry sorbent into a gas stream, and preferably by passing a gas flow through a spray dryer-absorber to disperse the sorbent in a bag filter before using fossil fuel pollutants in the fuel combustion system or when operating in an unsteady state.

During fuel combustion, the chemical energy in the fuel is converted into heat of heating, which can be used in various forms for a variety of applications. Fuels used in the combustion process include a wide range of solid, liquid and gaseous substances, including coal, oil (diesel, No. 2, Bunker C or No. 6), natural gas, wood, tire covers, biomass and the like. .

Burning converts fuel into a large number of chemical compounds. The main products of complete fuel combustion are water (H ₂ O) and carbon dioxide (CO ₂ ). However, other reactions occurring during the combustion of fuel with chemical components in the fuel result in the formation of undesirable by-products. Depending on the fuel used, such by-products may include particulate matter (e.g. fly ash), acid gases such as sulfur oxides (SO _x ) or nitrogen oxides (NO _x ), metals such as mercury or arsenic, carbon monoxide (CO) ) and hydrocarbons (HC). Emission levels for many of these by-products are regulated by government agencies such as the US Environmental Protection Agency (EPA).

There are several different technologies for removing such by-products from flue gases. In one method, known as spray drying chemical absorption or dry gas scrubbing, an aqueous alkaline solution or suspension that has been finely atomized is injected into the hot flue gases downstream of the combustion chamber in which the fuel was burned. The alkaline reagent reacts with contaminants and solid particles form. Water evaporates and cools the hot flue gases. Exhausted flue gas is typically characterized by a moisture content in the range of from about 10% to about 15%. After that, the flue gases pass into a device for collecting particulate matter, generally a bag filter, where particulate matter is removed from the flue gas, which is then sent to the chimney.

In the case of starting a fuel combustion system, such as a steam boiler having a furnace, from cold conditions, such as ambient temperatures, the furnace will typically burn natural gas or diesel fuel (No. 2) to “heat” the steam boiler before switching to coal. Before you can start burning coal, you need a furnace temperature in the range of about 400 ° F to about 500 ° F. Due to various start-up conditions and safety requirements, the furnace can start and stop several times before reaching stationary operation. Depending on the problems encountered, a full launch can take any period of time ranging from 8 hours up to 2 days to complete.

The dry gas purification desulfurization method does not work well at low temperatures. In particular, when using a spray absorber dryer so that the water can be completely vaporized, the temperature of the flue gas should typically be at least 220 ° F. During start-up, the temperature of the flue gases that pass into the spray dryer can be lower than this threshold temperature, however, SO _x and other pollutants are still formed. In addition to this, the furnace generally reaches a working temperature for coal in the range of 400 ° F to 500 ° F until the flue gas reaches 220 ° F in a spray dryer. As a result, this leads to higher SO _x emissions during startup. In addition, a bag filter generally requires 30 to 60 minutes of operation after starting the spray dryer to absorb a significant amount of alkaline material and achieve a significant degree of SO ₂ removal.

Previously, emission regulations did not include “abnormal” periods, such as start, stop and malfunction. However, due to the increase in restrictions established by regulatory enactments, it would be desirable to reduce such emissions. Ways that can reduce such emissions during startup would be very useful.

Disclosure of invention

This document describes various methods and systems for reducing SO _x emissions under non-stationary operating conditions, such as starting, stopping, or malfunctioning in a pollution control system that uses a dry gas scrubber for desulfurization. In short, dry calcium hydroxide powder is introduced into the flue gas, while the fuel combustion chamber is still at a temperature lower than normal operating temperature, such as when the fuel combustion chamber is heated and / or when the spray dryer is absorbed at a temperature lower than its threshold temperature of 220 ° F. The powder is introduced into the system, preferably upstream of the spray dryer. Thereafter, the resulting calcium hydroxide powder is collected in a downstream bag filter to obtain a filter cake that is suitable for reducing SO _x emissions.

In embodiments, a method is described for reducing emissions from fuel combustion during abnormal periods in a fuel combustion system, such as starting a fuel combustion chamber in a fuel combustion system. The fuel combustion system has a gas path that extends from the fuel combustion chamber through the spray dryer-absorber to a bag filter located downstream of the spray dryer-absorber. A carrier gas (which may be flue gas) flows through the gas path. The dry calcium hydroxide powder is mixed with a conveying gas, usually air, and pneumatically transferred to the inlet position, located downstream of the fuel combustion chamber and upstream of the bag filter, where the dry calcium hydroxide powder is blown and mixed with a carrier gas ( for example, as a result of input) in the gas path. To humidify and reduce the temperature of the carrier gas in a spray dryer, water is sprayed into the carrier gas. Calcium hydroxide powder precipitates in a bag filter to form a filter cake, which reduces emissions from fuel combustion. Sedimentation on the filter occurs without spraying the milk of lime in a spray dryer, which can only occur at temperatures in excess of 220 ° F.

In specific embodiments, no liquid is added to the carrier gas between the insertion position and the spray dryer-absorber.

The water sprayed into the carrier gas in the spray dryer absorber may come from a recirculation system to recycle the solids from the bag filter. Due to the recirculation of a smaller amount of solids at startup, the recirculation system can provide either water or suspension.

Sometimes the gas path passes through an air pre-heater located between the fuel combustion chamber and the spray dryer-absorber. The inlet position can be located between the air pre-heater and the spray dryer-absorber. Alternatively, the entry position is located upstream of the air pre-heater. A device for collecting particulate matter can also be located between the air pre-heater and the spray dryer-absorber, with the input position located downstream of the particulate collecting device.

The inlet position can also be located between the spray dryer-absorber and the bag filter.

The bag filter located downstream of the spray dryer absorber may be a fabric filter with pulse cleaning or a fabric filter with reverse gas purge.

In some abnormal conditions, such as a malfunction, the water sprayed in the spray dryer can be in the form of plain water (i.e. H ₂ O) or in the form of milk of lime (i.e. water plus calcium hydroxide). Water can also come from a recirculation system for recirculating solids from a bag filter or through auxiliary nozzles in the case of a non-working fine spraying means. In some embodiments, the flue gas entering the spray dryer absorber has a temperature in the range of about 140 ° F to about 210 ° F. In general, the carrier gas entering the spray dryer absorber has a temperature of less than 220 ° F.

The combustion chamber may have a temperature lower than 400 ° F. In embodiments, the carrier gas is flue gas exiting the fuel combustion chamber, especially during startup of the fuel combustion chamber. However, the methods can also be used during shutdown, that is, when the temperature of the carrier gas decreases over time.

It also describes a method for capturing pollutants in flue gases obtained by using a fuel combustion chamber in an unsteady state. The dry powder of calcium hydroxide is mixed with the flue gases in the inlet position, located downstream of the fuel combustion chamber and upstream of the spray dryer. To moisten and reduce the temperature of the flue gases in a spray dryer-absorber, water is sprayed into the flue gases. After that, the flue gases are passed through a bag filter located downstream of the spray dryer-absorber. Calcium hydroxide powder precipitates in a bag filter to form a filter cake that traps contaminants in the flue gas.

Data and other non-limiting characteristics are more specifically described below.

Brief Description of the Drawings

The following is a brief description of the drawings, which are presented for the purpose of illustrating the examples of embodiments described herein, and not for the purpose of limiting them.

Figure 1 is a diagram illustrating a conventional steam boiler with a dry desulfurization system.

Figure 2 is a diagram illustrating a fuel combustion system with a dry desulfurization system and a calcium hydroxide powder injection system in accordance with the present invention.

Figure 3 is an illustration of a filter sleeve in a pulse filter cloth filter.

Figure 4 is a view in local section of a spray dryer absorber.

Figure 5 is an illustration of the main components of the dry sorbent injection system.

Figure 6 is a graph of emissions versus time, showing actual emissions in the presence of calcium hydroxide input and estimated emissions in the absence of calcium hydroxide input.

Figure 7 is a general process diagram illustrating the methods of the present invention.

The implementation of the invention

A more complete understanding of the components, methods, and apparatus described herein can be obtained by referring to the accompanying drawings. These figures are merely schematic representations based on the convenience and ease of demonstrating the present description of the invention, and therefore are not intended to indicate the relative sizes and dimensions of the devices or their components and / or to determine or limit the scope of illustrative embodiments.

Despite the use of specific terms in the following description of the invention for the sake of clarity, these terms are intended only to indicate the specific structure of the embodiments selected for illustration in the drawings, and are not intended to define or limit the scope of the invention. In the drawings and in the following description of the invention, it is necessary to understand that such numerals refer to components with a similar function.

The singular forms “one,” “certain,” and “given” include plural matches, unless the context clearly dictates another.

In accordance with the use in the description of the invention and in the claims, the term “comprising” may also include embodiments of “consisting of” and “essentially consisting of”.

All ranges described herein are inclusive of the limit values and are independently combined (for example, the "250 ° F to 400 ° F" range includes the limit values of 250 ° F and 400 ° F and all intermediate values). The range boundary values and any values described herein are not limited to the exact range or value; they are inaccurate enough to include values that approximately display these ranges and / or values.

As used herein, an approximation language may be used to modify any quantitative representation that may vary without obtaining, as a result of a change in the basic function to which it relates. Accordingly, a value modified by a term or terms, such as “approximately” and “essentially”, may in some cases not be limited to the exact value indicated. In at least some cases, the approximation language may correspond to the accuracy of a device for measuring a given value. The “approximately” modifier should also be considered a description of the range defined by the absolute values in the form of two boundary values. For example, the expression “from about 2 to about 4” also describes the range “from 2 to 4”.

The term "hydrated lime" refers to calcium hydroxide, also known as Ca (OH) ₂ . The term “hydrated” as used herein does not mean that molecular water is present.

The term “milk of lime” is used to mean a mixture of calcium hydroxide and water. Other calcium sorbents include, for example, limestone or quicklime. The term "limestone" refers to calcium carbonate, also known as CaCO ₃ . The term "quicklime" refers to calcium oxide CaO.

The present description of the invention refers to components that are located “upstream” and “downstream” from other components. These terms are relative to another named component. This component will be located “upstream” from the named component if the path of the flow passes through this component before passing through the named component. Similarly, a given component will be located “downstream” from the named component if the flow path will pass through this component after passing through the named component.

The present invention relates to various methods and systems for reducing SO _x emissions under non-stationary conditions (i.e., during abnormal or abnormal conditions) in a pollution control system that uses a dry gas scrubber for desulfurization. In a very general case, flue gases are generated using a fuel combustion system comprising a combustion chamber in which fuel is burned. After that, dry calcium hydroxide powder is introduced into the flue gases, while the fuel combustion chamber is operated in an unsteady state. The powder is introduced upstream of the spray dryer. Thereafter, the resulting calcium hydroxide powder is collected in a downstream bag filter to form a filter cake that is suitable for reducing SO _x emissions.

In the general case, it is believed that such methods can be used in any system in which fuel is burned. Burning fuel can be used for any purpose, for example, to generate energy, produce a specific product, or simply to burn a given fuel. Examples of fuel combustion systems in which the present methods may be used include power generation systems that use a steam boiler having a firebox as a fuel combustion chamber; cement kilns; electric arc furnaces; glass melting furnaces; metal-smelting furnaces (copper, gold, tin and the like); granulating kilns; blast furnaces; coke oven batteries; chemical flame heaters; oil refining furnaces; and incinerators (for medical waste, municipal solid waste, and the like). The term “fuel combustion chamber” is used herein to refer to a specific structure in the system in which the fuel is burned.

Figure 1 generally illustrates one example of an energy generation system with a steam boiler 100 and a desulfurization system 110 located downstream. Fossil fuels 112, such as coal from a coal mill 111, and air 114 are burned in the furnace 105, which results in the formation of flue gases 120. Flue gases 120 pass through an economizer 116, which is used to preheat the water used in the steam boiler to produce water vapor and flue gas cooling 120. Other heat transfer surfaces located upstream of the economizer 116 are not shown. After that, the flue gas 120 enters the selective catalytic reduction system (SCR) 130, which may or may not be available to remove 120 nitrogen oxides (NO _x ) from the flue gases. After that, the flue gases 120 pass through an air preheater 140 to further cool the flue gases 120 and heat the air 114 entering the furnace 105. After passing through the air preheater 140, the flue gases 120 typically have a temperature in the range of from about 250 to about 400 ° F. (from 121 to 204 ° C). Occasionally, flue gases 120 then pass through a particulate collection device 150 for collecting fly ash and other large particles. Flue gases continue to flow into a dry gas scrubber or spray dryer / absorber 160. There, an atomized alkaline slurry 162 is injected into the flue gas to react with sulfur oxides (SO _x ) and to further cool the flue gas 120 to a temperature in the range of about 140 to approximately 210 ° F (60 to 99 ° C). The water in the suspension evaporates, and the resulting cleaned and particle-loaded flue gas 120 is transported to a particulate collecting device 170, such as a bag filter or electrostatic precipitator, to remove said particles from the flue gas 120. After that, the cleaned flue gas 120 is sent to the chimney 180. If desired, a recirculation stream 172 from the particulate collecting device 170 may be used to collect alkaline particles from the bag filter and mix them with water 176 in the recirculation tank 180 to obtain an alkaline suspension 162 which is used in the spray dryer 160. Alternatively, a fresh slurry 164 can be used in the spray dryer 160. Particles can also be removed from the solid collection device 170 particles for disposal, which in this case is indicated by the reference position 174.

Again, the temperature of the flue gas in the spray dryer must be at least 220 ° F before desulfurization activity can begin to occur. The only source of heat is the flue gas itself. In the case where the fuel combustion system is cold or, in other words, when the fuel combustion chamber is at ambient temperature, the temperature in the spray dryer-absorber will be insufficient. During start-up, shutdown, malfunction, or other abnormal periods, the fuel combustion chamber may emit SO _x and other pollutants while the flue gases are at a temperature lower than 220 ° F, and thus the dry gas scrubber does not can be used to desulfurize or remove other contaminants. This is an undesirable result.

In the methods of the present invention, desulfurization activity may occur in a bag filter during periods when the temperature in the spray dryer is too low to use an alkaline suspension. The term “non-stationary operating conditions” is used herein to mean periods when the temperature in the spray dryer absorber is less than 220 ° F (approximately 104 ° C). Such conditions may occur during abnormal periods such as starting, stopping and some malfunctions. During these conditions, calcium hydroxide precipitates in a bag filter, which provides and improves highly efficient acid removal. In this regard, the flue gas must pass through a filter cake formed on the filter in a bag filter, which provides direct contact between the flue gas and the alkaline product based on calcium hydroxide and promotes the absorption of vapor-phase acid gases (such as SO _x ) in the flue gas by filter. More generally, the present methods can be used to remove particulate matter from flue gases.

One condition for operating in an unsteady state, as provided for in the present description of the invention, occurs during the start of a fuel combustion system. During start-up, the combustion chamber and the desulfurization system (including the spray dryer / absorber) and the gases inside these components are at ambient temperatures. Temperatures increase until the combustion chamber reaches its stationary operating temperature, which can go up to 1000 ° F. In other words, during startup, the temperature of the flue gases exiting the fuel combustion chamber increases over time.

In addition to this, during start-up, while the temperature of the flue gas in the spray dryer absorber rises to its stationary operating temperature, only a limited amount of calcium hydroxide can be finely sprayed into the flue gas using a fine spray gun spray dryer absorber. This is due to the fact that, at lower flue gas temperatures, less water is required to cool the flue gases, and this cooling should not completely saturate the flue gases. In addition, there is a practical limit to the amount of alkaline sorbent in any slurry, since the alkaline slurry typically exhibits high viscosity once the slurry falls in the range of about 25% to about 35% solids, and this causes pumping problems which limit the degree of input of the alkaline sorbent through the means for fine atomization. Adding a dry powder of calcium hydroxide to the flue gases upstream of the spray dryer absorber avoids this viscosity problem. The necessary alkalinity can be added to the flue gas in the absence of an aqueous component of the suspension, which must be evaporated. It should be noted that water can still be added by methods other than the fine atomization means for evaporation, and can be transferred to a bag filter.

Another condition of operation in an unsteady state, provided for in the present description of the invention, occurs during shutdown of the fuel combustion system. During shutdown, the temperature of the fuel combustion chamber is intentionally reduced from the temperature of operation in a stationary state up to ambient temperatures. In other words, during a stop, the temperature of the flue gases exiting the fuel combustion chamber decreases over time. It should be noted that one reason for stopping may relate to maintenance purposes, such as cleaning and replacing the filter bags used in the bag filter itself. The filter cake present on the filter bags may be responsible for removing up to 63% of the SO ₂ entering the fabric filter. However, after cleaning or replacement, this desulfurization ability is absent until the filter cake is recovered. In accordance with the methods of the present invention, the filter cake can be recovered earlier and faster in comparison with what is expected to be possible to spray the alkaline suspension in the spray dryer-absorber, which results in a faster recovery of desulfurization ability and a faster reduction in total emissions. The goal is to pre-coat calcium hydroxide coatings on the filter bags before flue gas emissions exit the desulfurization system.

The third non-stationary operating condition provided in the present description of the invention occurs during a malfunction of the fuel combustion system, which results in a decrease in the gas temperature in the spray dryer-absorber below 220 ° F. Such a malfunction in the general case is very serious given the large temperature drop and differs from stopping mainly by unintentionality and / or by the fact that it does not lead to a temperature drop up to ambient temperatures.

Figure 2 generally illustrates one example of a system of the present invention comprising a fuel combustion system 200, a downstream desulfurization system 210, and a dry calcium hydroxide powder input system 290. Like figure 1, air 214 and coal 212 from a coal-grinding mill 211 are burned in a fuel combustion chamber 205, which results in the formation of flue gases 220. In essence, flue gases are a carrier gas that travels along the gas path. Flue gases pass economizer 216 (other heat transfer surfaces upstream of the economizer are not shown) and an SCR system 230, which may or may not exist, and which removes NO _x from the flue gases. The flue gas passes through the air pre-heater 240 and continues to flow into the spray dryer-absorber 260. Optionally, an optional particulate collector 250 may be arranged between the air pre-heater 240 and the spray dryer-absorber 260 to collect fly ash and other large particles. In a spray dryer 260 for purification and cooling of flue gases, a finely divided alkaline suspension 262, such as milk of lime, is injected into the flue gases 220. The resulting cleaned and particle-loaded flue gas 220 is passed into a bag filter 270 to remove said particles from the flue gas. After that, the cleaned flue gases 220 are sent to the chimney 280. If desired, a recirculation stream 272 from the bag filter 270 can be used to collect unreacted alkaline particles from the bag filter and mix them with water 276 in the recirculation tank 280 in order to obtain an alkaline suspension 262, which used in a spray dryer absorber. Alternatively, a fresh slurry 264 may be used in the spray dryer absorber 260. Particles from the baghouse can also be disposed of, which is indicated by reference numeral 274 in this case.

The fuel combustion chamber 205 is located upstream of the air preheater 240, which is located upstream of the spray dryer-absorber 260. A bag filter 270 is located downstream of the spray dryer-absorber 260. In other words, the spray dryer the absorber 260 is located between the air pre-heater 240 and the bag filter 270. The SCR system 230, if any, is located between the fuel combustion chamber 205 and the pre-heater oil 240 air.

The present methods include a gas path 220 between the fuel combustion system and the desulfurization system. A carrier gas, such as flue gas or ordinary air, flows through the gas path. Dry powder of calcium hydroxide is introduced into the carrier gas in the inlet position located downstream from the fuel combustion chamber 205 and upstream from the bag filter 270. To cool and humidify the flue gases in a spray dryer-absorber 260, water is sprayed into the carrier gas. After that, calcium hydroxide powder is deposited in a bag filter 270 to obtain a filter cake, which is used to reduce emissions.

Filter cake can be obtained before adding fuel producing relatively large amounts of pollutants (e.g. coal) to the combustion chamber 205 or, in other words, while using relatively clean fuel (e.g. natural gas) to heat the combustion chamber fuel. As envisioned, calcium hydroxide powder could be introduced in a range of 60 to 90 minutes earlier than the spray dryer-absorber 260 could otherwise be started, so that alkalinity could be added to more quickly coat the bag filter 270 and an earlier onset of desulfurization. In other words, a filter cake can be obtained before spraying water or a suspension (i.e., milk of lime, a recirculation suspension, or a combination thereof) in a spray dryer 260.

System 290 input dry powder of calcium hydroxide contains a source 292 supply of calcium hydroxide. It is assumed that calcium hydroxide powder can be introduced into the desulfurization system in three different positions A, B, C. All three of these input positions are located downstream of the fuel combustion chamber 205 and upstream of the bag filter 270. In particular, to maintain hydrated lime stability, the temperature of the flue gas / carrier gas should be less than 1000 ° F.

The first inlet position A is located downstream from the air pre-heater 240 and upstream from the spray dryer-absorber 260. In other words, the inlet position A is located between the air pre-heater 240 and the spray dryer-absorber 260. Upstream from position A of the input may be an optional device 250 for collecting particulate matter. In some embodiments, the flue gas entering the spray absorber dryer (i.e., in the input position A) has a temperature in the range of from about 140 ° F to about 210 ° F or from about 150 ° F to about 200 ° F or approximately 160 ° F to approximately 170 ° F.

The second position B of the input is located downstream from the fuel combustion chamber 205 and higher upstream from the air pre-heater 240. The second entry position B may also be described as being downstream of the SCR system 230.

The third input position C is located downstream of the spray dryer-absorber 260. In other words, the input position C is located between the spray dryer-absorber 260 and the bag filter 270.

The dry calcium hydroxide powder can also be simultaneously introduced at various points above. Turning again to FIG. 2, it can be said that the water that is sprayed in the spray dryer-absorber 260 may come from a separate water source or, in some embodiments, may come from a recirculation system 280. During start-up, through the recirculation stream 272, the solid enters in an amount ranging from small to zero, so that the stream 262 is essentially water.

The optional particulate collection device 250 in various embodiments may be either an electrostatic precipitator (ESO) or a bag filter. Various types of bag filters are known in the art, for example, a gas filter with a reverse gas purge, a fabric filter with shaking during pumping and a fabric filter with pulse cleaning.

The bag filter 270, located downstream of the spray dryer-absorber 260, is preferably a pulse filter fabric (PJFF) or a gas filter with a gas backflow. In this regard, the bag filter in this position is preferable in comparison with the ECO device because of the better ability to desulfurize the bag filter in comparison with the ECO device. In other words, a bag filter can trap pollutants that are in the vapor phase, while an ESO device only traps particles and does not significantly trap vapor-phase pollutants. In the general case, all the flue gases entering the bag filter 270 must pass through the filter cake, so that acid gases such as SO ₂ , SO ₃ and HCl can be removed.

Figure 3 is a schematic illustration of a pulse filter cloth filter. A bag filter generally contains several compartments, with each compartment containing up to several hundred long vertically supported, small diameter fabric hoses. In a pulse filter fabric filter (PJFF), sleeves 320 hang from the tube sheet 330. Flue gases containing solid particles flow from the outside of the sleeve (as indicated by solid colored arrows) into the sleeve (as indicated by outlined arrows). Flue gases pass through the porous material of the bag, leaving solid particles behind to form a precipitate 340 on the filter on the outside of the bag. A pulse of compressed air can be directed from the open top 322 into the sleeve, which will create a shock wave that travels down the length of the sleeve and knocks the filter cake out of place.

Calcium hydroxide is used because its salt is insoluble in water. In contrast, sodium sorbents are generally soluble and thus less desirable. In addition, calcium hydroxide is safer than quicklime, which generates heat when combined with water.

As the applicants have determined, the reactivity of calcium hydroxide powder is comparable to the reactivity of calcium hydroxide in milk of lime. This makes possible the acceptable functioning of the dry desulfurization system under various conditions in an unsteady state. In this case, before starting, calcium hydroxide powder can be added before it is possible to introduce sufficient amount of milk of lime to obtain a significant filter cake, which thus ensures a much earlier achievement of acceptable acid gas emissions in comparison with what will be possible otherwise.

Generally, it is more desirable to inject the calcium dioxide powder upstream of the spray dryer-absorber 260 (i.e., the entry positions A or B), since the spray dryer-absorber facilitates proper dispersion of the powder throughout the bag filter 270. Figure 4 is a view with a local section through a spray dryer 400, commonly used in desulfurization systems. The spray dryer absorber typically has a truncated cone body 410, with the top of the cone located at the bottom of the absorber. However, spray dryers can also have a flat bottom instead of a cone. The flue gases 420 coming from the air heater can be split into two streams 422, 424, although this is not always the case and is not necessary for the present invention. One stream 422 is directed to the upper gas dispersant 430, which has an annular shape. Another stream 424 is directed to a lower gas dispersant 440. The fine spray means 450 passes through the center of the roof of the absorber housing and sprayes the milk of lime into the flue gases. Flue gases enter the spray dryer 400 through gas dispersers. The spray dryer-absorber is designed to guarantee good mixing of the flue gas with the suspension and has a size that provides sufficient residence time for drying the suspension in order to obtain a free-flowing solid without internal deposits. However, with regard to start-up, mixing and turbulence imparted to the calcium hydroxide powder by the spray dryer-absorber guarantee better dispersion of calcium hydroxide throughout the filter bags in the bag filter. When using a fine spraying agent 450, water is added to a dry calcium hydroxide powder in a spray dryer dryer to form a suspension of calcium hydroxide. Water does not necessarily completely evaporate, especially at low temperatures, and it is required in a bag filter to achieve its complete desulfurization ability on the filter cake, since the mechanism of the SO ₂ absorption reaction requires the presence of molecular water. The evaporated suspension of calcium hydroxide leaves the spray dryer through an outlet 460 and passes into a bag filter.

5 is a schematic diagram for a typical dry sorbent feed system for hydrated lime. Hydrated lime can be delivered 510 either by truck or by rail (in this case, unloading of a truck is illustrated). Ambient air 512 is blown into the truck to trap hydrated lime and transfer the reagent to storage silo 520. The reagent flows from the silo 520 for storage through a sequence of valves 522, feeders 524 and hoppers 526, 528 to the rotary gateway 530, where the reagent is mixed with transporting gas 540 for pneumatic transfer to the position of entry into the gas path (see figure 2). The transport gas, typically air, is supplied by transport air blowers 542 that bypass the transport gas through air coolers 544 to reduce air temperature in order to prevent premature calcination of the reactant. It should be noted that in the present system, no liquid is introduced into the gas path between the position of the inlet and the spray dryer-absorber. This is in conflict with the prior art systems, where solutions and suspensions were introduced into the flue gases upstream of the wet or dry scrubber; see, for example, US Patent No. 6126910 to Wilhelm. This also contradicts the system where dry calcium sorbent was introduced and then moistened using water in the piping system; see, for example, US Pat. No. 5,165,903 to Hunt. In prior art data systems, the desired goal is to remove selected pollutants from the flue gas before entering the desulfurization system. In contrast, the purpose of the present methods is to provide an alternative source of alkaline reagent (hydrated lime), to increase the concentration of hydrated lime in a spray dryer absorber, and to apply calcium hydroxide coating in the bag filter to provide desulfurization and improved desulfurization ability. The addition of water or liquid to the spray dryer-absorber as a result may lead to an undesirable state in the form of precipitation of calcium hydroxide from the gas and the inability to transport it to the bag filter.

The methods of the present invention improve the ability of a desulfurization system to respond and function within acceptable levels of acid gas emissions as a result of a method for responding timely to variations in emission levels. One cross-cutting issue in maintaining the operation of fuel combustion systems is the time required to resolve this problem. Calcium hydroxide powder can be added quickly and a good response will be obtained. The methods also offer a dry sorbent that does not require the addition of water to the process.

Figure 7 is a general process diagram illustrating the methods of the present invention. The fuel combustion system 700 comprises a fuel combustion chamber 705 in which fuel is burned and results in the formation of flue gases. Flue gases pass through a gas passage 720 through a spray dryer-absorber 760 to a bag filter 770 located downstream of the spray dryer-absorber. A dry calcium hydroxide powder is mixed into the flue gas (gas path 720) between the fuel combustion chamber 705 and the bag filter 770. For example, calcium hydroxide powder can be added upstream of the spray dryer (reference 794) or lower by upstream of the spray dryer / absorber (reference numeral 796). Inside the spray dryer 760, water is sprayed into the flue gas to humidify and cool the flue gas (reference numeral 762). Flue gases are passed into a bag filter 770. Calcium hydroxide traps pollutants or particulate matter in flue gases. The cleaned flue gases are sent to a chimney 780 or the like to be released into the atmosphere.

The layout for the practical implementation of the methods of this description of the invention are within the knowledge of one of ordinary skill in the art. Also in the general case, the valves, pipe wiring, sensors, connectors and fittings required to implement the practical implementation of these methods are commercially available.

Example

The 120 MW power plant (gross) (Megawatt-gross) had the equipment layout shown in Figure 2. The use of calcium hydroxide powder was carried out during startup and in order to replace the milk of lime. Calcium hydroxide powder was introduced at positions A and C of the input. The actual emissions of SO ₂ through the chimney are shown in Figure 6. The y-axis represents the amount of SO ₂ emissions in units of pounds / MBTU (pounds per million British thermal units). The x axis represents the time of day, that is, from midnight (0:00) to 12:00 in the afternoon. For comparison, the adjusted SO ₂ emission limit through the chimney of 0.09 lb / MBT is shown. Two lines are demonstrated: one for actual emissions and one for estimated emissions in the absence of calcium hydroxide powder input. It should be noted that in this figure, the launch attempt was made three times: approximately at 12:30 until noon, at approximately 2:45 to noon, and approximately at 5:45 to noon.

The present invention has been disclosed with reference to examples of embodiments. Obviously, after reading and understanding the preceding detailed description of the invention, modifications and changes to other embodiments will become clear. It is intended that the present invention be understood to include all such modifications and changes to the extent that they fall within the scope of the appended claims or their equivalents.

Claims (32)

1. A method of reducing emissions from fuel combustion, obtained under non-stationary operating conditions in a fuel combustion system having a gas path that passes from the fuel combustion chamber through the spray dryer-absorber to a bag filter located downstream of the spray dryer an absorber, the method comprising: mixing dry calcium hydroxide powder into the carrier gas in the gas path in the inlet position located downstream of the fuel combustion chamber and higher upstream of the bag filter; spraying water into the carrier gas in a spray dryer to humidify and reduce the temperature of the carrier gas; and the precipitation of calcium hydroxide powder in a bag filter to obtain a filter cake that reduces emissions from fuel combustion, moreover, the temperature in the spray dryer absorber is less than 220 ° F. 2. The method according to claim 1, in which between the position of the input and the spray dryer-absorber in the carrier gas is not added liquid. 3. The method of claim 1, wherein the water sprayed into the carrier gas is supplied from a recirculation system to recirculate the solid from the baghouse. 4. The method according to claim 1, wherein the gas path passes through an air pre-heater located between the fuel combustion chamber and the spray dryer-absorber. 5. The method according to claim 4, in which the input position is located between the air pre-heater and the spray dryer-absorber. 6. The method according to p. 4, in which the input position is located upstream from the air pre-heater. 7. The method according to claim 4, further comprising a device for collecting particulate matter between the air pre-heater and the spray dryer-absorber. 8. The method according to p. 7, in which the input position is located downstream from the device for collecting particulate matter. 9. The method according to claim 1, in which the input position is located between the spray dryer-absorber and the bag filter. 10. The method according to p. 1, in which the bag filter is a fabric filter with pulse cleaning, fabric filter with shaking when pumping or fabric filter with reverse gas purge. 11. The method according to p. 1, in which the water sprayed in the spray dryer absorber is sprayed through auxiliary nozzles, and the means for fine spraying the spray dryer absorber does not function. 12. The method according to claim 1, in which the carrier gas entering the spray dryer-absorber has a temperature in the range from about 140 ° F to about 210 ° F. 13. The method according to p. 1, in which the carrier gas entering the spray dryer-absorber has a temperature of from less than 220 ° F. 14. The method of claim 1, wherein the fuel combustion chamber has a temperature lower than 400 ° F. 15. The method of claim 1, wherein the carrier gas is flue gas exiting the fuel combustion chamber during the start of the fuel combustion chamber. 16. The method of claim 1, wherein the temperature of the carrier gas decreases over time. 17. The method according to p. 1, in which the condition for functioning in an unsteady state is a malfunction in the gas path. 18. The method according to p. 1, in which the fuel combustion system is selected from steam boilers, kilns, furnaces, metal furnaces, kilns, batteries, heaters, muffle furnaces or incinerators. 19. A method for capturing pollutants in flue gases obtained by using a fuel combustion chamber under operating conditions in an unsteady state, the method comprising: mixing dry calcium hydroxide powder into the flue gas in the inlet position located downstream of the fuel combustion chamber and upstream of the spray dryer; spraying water into the flue gases in a spray dryer to humidify and reduce the temperature of the flue gases; and flue gas by-pass through a bag filter located downstream of the spray dryer, where calcium hydroxide powder is deposited in the bag filter to form a filter cake that traps contaminants in the flue gas, moreover, the temperature in the spray dryer absorber is less than 220 ° F. 20. The method according to p. 19, in which between the position of the input and the spray dryer-absorber in the flue gases do not add liquid. 21. The method according to p. 19, in which the flue gases entering the spray dryer absorber have a temperature in the range from about 140 ° F to about 210 ° F. 22. The method according to p. 19, in which the flue gases entering the spray dryer absorber, have a temperature of from less than 220 ° F. 23. The method of claim 19, wherein the flue gases exiting the fuel combustion chamber have a temperature lower than 400 ° F. 24. The method of claim 19, wherein the fuel combustion chamber is located in a fuel combustion system selected from steam boilers, kilns, furnaces, metal furnaces, kilns, batteries, heaters, muffle furnaces, or waste incinerators.

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